High efficiency inline fluid heater

ABSTRACT

A heater for heating flowing fluid. The heater comprises a cylindrical heating chamber comprising an inlet at one base and on outlet at a second base. The flowing fluid traverses parallel to the chamber. A resistive heating element is received in the chamber wherein the resistive heating element comprises a rectangular base. The rectangular base comprises a long axis and short axis and the long axis is parallel to the chamber. The base comprises a multiplicity of first slots on one side and a multiplicity of second slots on a second side. A double helix filament is received in the first slots and the second slots.

TECHNICAL FIELD

[0001] The present invention is related to an inline fluid heater. Morespecifically, the present invention is related to a highly efficientinline fluid heater particularly adapted for heating flowing air.

BACKGROUND

[0002] Devices for heating fluid, particularly flowing air, are commonlyemployed in many areas of commerce. Hair dryers, heat guns and similardevices, for example, utilize a resistance based heating element to heatair flowing past the heating element.

[0003] Electrical resistance heaters typically comprise a support with aresistance filament, particularly a wire filament, secured thereto. Eachend of the wire filament is attached to a power supply. U.S. Pat. No.5,298,723 provides a representative resistive heating element comprisinga helical coil filament wrapped around a ceramic or insulating comb. Airflows past the helical coils allowing heat to transfer via convectionfrom the heated coils to the air. One deficiency with the heatingelement of U.S. Pat. No. 5,298,723 is the poor efficiency of heattransfer from the coils to the flowing air.

[0004] A common problem associated with resistance type heaters is theproximity of the terminals. It is highly desirable to have the terminalsat the cool side of the heating element. This configuration acts to coolthe terminal, and contacts, and insures that they do not becomeoverheated. If the terminal is towards the exit side of the heatingelement the terminal is heated by conduction heating, from the resistiveelement, and from convection from the heated air. Therefore, coiledresistive heating elements have generally been considered inappropriatefor use in enclosed tube heaters where air passes into one end of atube, in the proximity of a terminal, and out the other end, also nearthe proximity of a second terminal.

[0005] Yet another problem with many existing resistive heating elementsis the poor interaction between the heated coils and the flowing air.Resistive heating elements are typically designed such that air flowssubstantially perpendicular to the rotational axis of the cylinderdefined by the coils. Therefore, assuming linear air flow with a flatvelocity profile, the air can only contact the coil in two locations oneach side of the support. While this model is simplified, the principleclearly indicates that the contact between flowing air and a heatedresistive element is minimal. If the air flow is altered to be parallelto the rotational screw axis of the coil the previously stated problemof terminal heating occurs.

[0006] One of ordinary skill in the art has therefore been forced tochoose between two undesirable choices when employing helical resistiveheating elements. One choice leads to terminal heating and high rates offailure. The other choice leads to inefficient heating. Neither of theseis acceptable.

[0007] One solution to the problems described above is the use ofceramic, honeycomb type, supports with filaments interwoven therein.While this solution mitigates the deficiencies of the elements describedin U.S. Pat. No. 5,298,723 other problems occur. Ceramic heaters havethe advantage of high efficiency with regards to heat transfer.Honeycomb based ceramic heaters are currently considered in the art tobe the heaters of choice due, in part, to the high efficiency. Honeycombtype ceramic cores are expensive to manufacture and the air flow througha honeycomb type ceramic core is, at least partially, blocked therebyrequiring higher air pressures at the entrance of the heater to achievethe desired air flow at the exit of the heater. The reliance on higherentrance pressure increases the operating cost of honeycomb basedheating elements.

[0008] Another widely used solution is the use of an elongated terminalrunning the length of the support. This solves the problem associatedwith connection overheating yet other problems occur. The contactbetween flowing air and the elongated terminal is poor thereby allowingthe elongated terminal to become overheated. Any local overheating in aresistive heater represents a potential area of failure. It is one goalof a resistive heating element design to provide consistent temperaturesover the entirety of the resistive element to avoid hot spots.

[0009] There is not an adequate heating element available in the artcapable of solving all of the problems described above.

[0010] There has been a long felt need in the art for a resistiveheating element which is highly efficient in transferring heat from theresistive element to the flowing fluid but which does not have thedeficiency associated with connection overheating. There has also been along felt desire to accomplish these tasks without the expense, andmanufacturing burdens, associated with ceramic, honeycomb type,resistive heating elements.

SUMMARY

[0011] It is an object of the present invention to provide a resistiveheating element capable of efficiently transferring heat from a coil toa fluid flowing thereby.

[0012] It is another object of the present invention to provide an inline heating element which has improved flow characteristics asdetermined by the pressure required to achieve a given flow volume.

[0013] It is another object of the present invention to provide aresistive heating element which is cost efficient to manufacture andwhich is less susceptible to failures due to localized hot spots in theresistive heating element.

[0014] It is another object of the present invention to provide aheating element, which meets the above described demands, withoutrequiring one, or both, connections to be subjected to heated flowingfluid.

[0015] Yet another advantage of the present invention is the improvedsupport of the heating element thereby decreasing the detrimentalproperties associated with coil instability.

[0016] These and other advantages, as will be realized, are provided inan inline heating element for heating a fluid. The heating elementcomprises an elongated tubular heating chamber comprising an inlet andan outlet opposite to the inlet. A heating element in received in theheating chamber. The heating element comprises an elongated basecomprising a first face, a second face parallel to the first face, afirst edge and a second edge. A coiled filament circumvents the basewherein the coiled filament comprises a first region wrapped clockwisearound the base and a second region wrapped counterclockwise around thebase. The first region terminates at a first terminal and the secondregion terminates at a second terminal. The elongated base is parallelto the elongated tubular heating element; and the fluid flows parallelto the base.

[0017] Yet another advantage is provided in an in line heating elementfor heating a fluid. The element comprises an elongated tubular heatingchamber comprising a long tube side and a short tube side, an inlet andan outlet opposite to the inlet. The fluid enters the inlet andtraverses parallel to the long tube side and exits the outlet as heatedfluid. A heating element is received by the heating chamber. The heatingelement comprises a rectangular base comprising long base sides andshort base sides wherein the long base sides are parallel to the longtube side. A continuous coiled resistive heating element circumventingthe base in a double helix wherein the double helix comprises arotational axis of symmetry and the rotational axis of symmetry isparallel to the long base.

[0018] A particular advantage is provided in a heater for heatingflowing fluid. The heater comprises a cylindrical heating chambercomprising an inlet at one base and on outlet at a second base. Theflowing fluid traverses parallel to the chamber. A resistive heatingelement is received in the chamber wherein the resistive heating elementcomprises a rectangular base. The rectangular base comprises a long axisand short axis and the long axis is parallel to the chamber. The basecomprises a multiplicity of first slots on one side and a multiplicityof second slots on a second side. A double helix filament is received inthe first slots and the second slots.

BRIEF DESCRIPTION OF DRAWINGS

[0019]FIG. 1 is partial cutaway view of an embodiment of the presentinvention.

[0020]FIG. 2 is a top view of the embodiment of FIG. 1 taken along line2-2 of FIG. 1.

[0021]FIG. 3 is a bottom view of the embodiment of FIG. 1 taken alongling 3-3 of FIG. 1.

[0022]FIG. 4 is an exploded view of an embodiment of the presentinvention.

[0023]FIG. 5 is a perspective partial cutaway view of a preferredheating element of the present invention.

[0024]FIG. 6 is a diagram representing the improved air flow with theinventive heating element.

[0025]FIG. 7 is a diagram representing the improved heating efficiencyof the inventive heating element.

[0026]FIG. 8 illustrates a base with progressive spacing of the slots.

DETAILED DESCRIPTION

[0027] The present invention relates to a resistive heating element forheating flowing fluid. The invention will be described with reference tothe various figures forming a part of this disclosure. The drawings arefor the purposes of discussion and are not intended to limit theinvention in any way.

[0028] An inline heater, in accordance with the present invention, isillustrated in FIG. 1, and generally indicated at 1. A top view of theinline heating element, taken along line 2-2 of FIG. 1, is provided inFIG. 2. A bottom view of the inline heating element, taken along line3-3 of FIG. 1, is provided in FIG. 3.

[0029] The inline heater, 1, comprises an inlet, 2, and outlet, 3. Theinlet, 2, receives the flowing fluid, 27, and preferably represents afitting which can be coupled to a supply line (not shown) by threads,pressure fittings, adhesives or the like. The manner in which the inletis coupled to the supply line is not limiting herein. Fluid passes intothe heating chamber, 4, and is forced into thermal conductive contactwith a heating element, 5. The heating element, 5, will be describedwith more detail herein. As the fluid transits through the heatingchamber, 4, the temperature of the fluid increases based on thetemperature of the heating element, 5, the effective contact with theheating element, 5, and the efficiency of heat transfer from the heatingelement, 5, and the fluid. Heated fluid, 28, exits the outlet, 3. Theoutlet, 3, may further comprise a nozzle to diffuse or concentrate theheated fluid, or the outlet may be coupled to a secondary supply linewherein heated air is transported to any number of devices such as awork site, assembly or processing equipment, second heater, etc.

[0030] Optional baffles in the inlet, 2, and/or outlet, 3, may beprovided to diffuse the air thereby mitigating the effect of flowchannels which can occur from the configuration of the supply line or todiffuse, or concentrate, the heated fluid as it exits the outlet.

[0031] An optional, but preferred, inlet couple, 8, may be attached tothe inlet, 2, or heating chamber, 4, by mating threads, welding,adhesive, friction fit, mating voids and protrusions, or the like. Theoutlet couple may also secure the heating element within the heatingchamber and may be an open ring. The inlet couple, 8, may also beintegral to the heating chamber, 4, or integral to the supply line. In apreferred embodiment the inlet couple, 8, comprises a terminal couple,9, through which the electrical terminals, 10, pass for connection to apower source. The terminal couple, 9, allows the electrical terminals,10, to be encased in a non-conducting conduit, such as flexible or fixedconduit, for protection from thermal or physical stresses if so desired.It is most desired that the terminal couple, 9, be isolated from thefluid flow.

[0032] An optional, but preferred, outlet couple, 11, allows the outletto be connected to a device for directing fluid flow. While not limitedthereto, the outlet couple, 11, may comprise, or be attached to, adiffuser or a concentrator. The outlet may also be connected to aconduit for transporting heated fluid to a secondary location; todetectors for monitoring the temperature, volume, velocity or otherproperties associated with the heated fluid; or to other devices aswould be commonly employed in connection with an outlet of a heatingelement.

[0033] An embodiment of the present invention is illustrated in explodedview in FIG. 4. In FIG. 4, the heating element, 5, comprises a base, 15,with a coiled filament, 16, circumventing the base in a double helicalconfiguration. The base, 15, is preferably an elongated planar elementcomprising a multiplicity of slots, 17 and 18, along each edge, 24. In apreferred embodiment the slots are, at least partially, aligned. Alignedslots are preferred however with small diameter units offset slots maybe preferred due to space limitations.

[0034] In a particularly preferred embodiment the separation betweenslots is progressively closer from inlet to outlet. In this embodiment,illustrated in FIG. 8, the helical coils are further apart on the inletside than on the outlet side. This configuration is preferable forbalancing the heat transfer with energy usage.

[0035] Referring again to FIG. 4, the slots receive the coiled filament,16. The coiled filament, 16, is received in offset fashion along thelength of the base. By offsetting each wrap one slot the coiledfilament, 16, forms a double helix thereby allowing the filament toterminate on the same side of the base, 15. The filaments are preferablyelectrically connected to electrical terminals, 10. By way ofclarification, starting with terminal 10 b, filament, 23, passes throughpassage void, 19, and then, as a coiled filament, is received by slot 18a. The coiled filament is wrapped counterclockwise (as viewed from theterminal end) and received by sequentially offset slots 17 b, 18 c, 17d, 18 e, 17 f, 18 g, 17 h, 18 i, 17 j, 18 k, 17 l, 18 m, 17 n . . . 18u. The coil is received by terminal slot, 21. After being received byoptional terminal slot, 21, the rotation of the wrap is clockwise (asviewed from the terminal end) and the coil is received by sequentiallyoffset slots 17 u, 18 t, 17 s, 18 r, 17 q, 18 p, 17 o, 18 n, 17 m . . .17 a. Filament, 23, then passes through passage void, 20, and is inelectrical connection with electrical terminal 10 a. Two passage voids,as illustrated in FIG. 8, may be employed to further secure the coil.The use of sequentially offset slots insures that each subsequent coilvisible on a planar surface of the base is separated, relative to thelength of the continuous resistive element. For example, each coilsegment, 16 a, from the clockwise wrap is separated from each adjacentcoil segment, 16 b, from the counterclockwise wrap. This double helixconfiguration greatly increases the efficiency of the heating elementwhich is unexpected in the art.

[0036] The passage voids, and terminal voids are optional, butpreferred. The passage voids allow the initial position of the filamentto be fixed prior to initiation of the wrapping procedure. The terminalvoid, likewise, insures that the wrap is secured prior to the returnwrap initiation.

[0037] Heating element, 5, is received by an optional, but preferred,liner, 6, which is in-turn, received in the heating chamber, 4. Theelectrical terminals, 10, preferably protruding through the optionalterminal couple, 9. Optional outlet couple, 11, and optional inletcouple, 8, secure the heating element into a fixed position within theheating chamber in an orientation which avoids coils contacting theinterior surface of the heating chamber. It is most preferred that theheating chamber comprise an interior liner, 6, which is both thermallyand electrically insulating. Preferably liners include flexible mica,thin mica sheets, quartz or ceramic. Flexible mica and thin mica sheetsare preferred due, in part, to the cost advantages versus quartz andceramic. Quartz and ceramic are excellent insulators and provide lessdegradation but the cost is prohibitive relative to mica for mostapplications. In one embodiment a ground wire, 7, is attached to theheating chamber as a safety feature.

[0038] A partial cut-away view of a preferred heating element, 5, isprovided in FIG. 5. In FIG. 5 a portion of the base is cut-away to allowa portion of the double helix coiled filament, 16, to be visualized. Thedouble helix comprises two parallel matching wraps with a common axis ofrotation and radius with each wrap offset such that they do not comeinto contact with each other. The face, 25, may comprise interior slots,26, to increase air flow channels if so desired. The rotational axis,29, is defined as the axis of rotation of the cylinder formed by theexterior of the double helix. The rotational axis is parallel to thelong axis of the rectangular base.

[0039] For the purposes of the present invention, an in line heatercomprises an elongated enclosed heating element wherein the lengththrough the heating chamber is longer than the cross-sectional diameterof the heating chamber.

[0040] It would be understood that the heating chamber is preferablyconfigured to insure that heating coils cannot easily come into contactwith the interior surface of the heating chamber. This may beaccomplished in several ways. In a preferred embodiment the heatingchamber has an insulating liner. In one embodiment, the depth of theslots is larger than the exterior diameter of the cylinder defined bythe coils. This insures that the base will contact the interior surfaceof the heating chamber prior to the coils coming into contact with theinterior surface of the heating chamber. The width of the base ispreferably wider than the outer diameter of the cylinder defined by thedouble helix. The width of the base is preferably chosen to be smallenough to be easily received by the heating chamber with minimal lateralmovement therein. A clearance of approximately 0.010″ to approximately0.015″ is most preferred.

[0041] The heating chamber is preferably cylindrical, more preferably aright cylinder with a preferably round base. Other shapes such astrigonal, square, pentagonal, hexagon or polygonal, are suitable fordemonstration of the teachings herein as are oblong configurations suchas elliptical. It is most preferred that the interior shape, anddimensions, be chosen in tandem with the size of the base to prohibitthe coils from contacting the interior of the heating chamber.

[0042] The resistance wire is chosen to maximize heat transfer. Thegreatest efficiency will be achieved when the primary mode of heattransfer is convection versus radiation. This implies that surfacetemperature must be kept relatively cool. It has been found that formost applications the heat flux (also referred to as watt density whichis watts per surface area of wires) should be kept under 100 W/m². Ifthe pitch of the coil is to small, convection is inhibited from theinability of the air to flow between adjacent turns. In addition, whenthe adjacent coil turns are in close proximity the wire surfacetemperature is maintained at a higher temperature from the radiationexchange between adjacent turns. Both scenarios result in lower heattransfer efficiency. Best results have been achieved when the coil helixpitch is in the range of 2.5 times wire diameter to 4 times wirediameter. The wire gauge can be selected for a desired watt density, andthe inside diameter (arbor diameter) can be varied to achieve thedesired coil helix pitch. In addition, to insure the coil does not sagor creep at high temperatures a coil of adequate stiffness can beachieved if the ratio of arbor diameter to wire diameter is less than10.

[0043] The base is any support material typically employed in electricalresistance heaters. Particularly preferred materials include mica,steatite, cordierite, quartz and ceramic with mica being most preferred.Mica is preferred due, in part, to the cost and ease of fabrication. Forhigher temperature, and higher strength applications ceramic ispreferable but the tooling required to form ceramic parts typicallylimits the desire to utilize ceramic.

[0044] The coiled filament is any material typically employed in themanufacture of resistant heaters. Resistive alloys are preferred.Particularly preferred resistive alloys include nickel chrome alloy andiron chrome aluminum alloys. Nickel chrome alloys are availablecommercially comprising approximately 12-25%, by weight, chromium;approximately 2.75-6%, by weight, aluminum and the balance iron. Apreferred example is Kanthal AF. Nickel chromium alloys are availablecommercially comprising from approximately 35-80%, by weight, nickel;approximately 16-20% chromium and minor portions of such materials assilicon, manganese, carbon, iron and sulfur. A particularly preferrednickel chromium alloy comprises approximately 80%, by weight, nickel andapproximately 20%,by weight chromium.

[0045] The in line heater of the present invention is particularlysuitable for heating air but other fluids can be heated utilizing theinventive heater without departing from the scope of the presentinvention. Gaseous fluids are more preferable than liquid fluids withair being the most preferred.

Experimental EXAMPLE 1

[0046] A heating element was prepared in accordance with the presentinvention. The heating element comprised an approximately 4.7 inch byapproximately 0.4 inch base with 21 slots cut along each side. A 27.9Ohm, 25 gauge (0.179″ diameter) Kanthal AF coil was wrapped in a doublehelix with an outside diameter (OD) of approximately 0.115″ and an arborof approximately 0.075″. As a control a conventional ceramic heatercomprising a 27.9 Ohm, 25 gauge (0.179″ diameter) Kanthal AF wire waswound in serpentine fashion through six holes of a ceramic coreassembly. The core assembly was approximately 4.315″ long comprising sixapproximately 0.110″ diameter holes on an approximately 0.285″ diameterbolt circle with an outer diameter of approximately 0.435″. Each heatingelement was subjected to an air flow analysis. The results are providedin FIG. 6. The results provided in FIG. 6 indicate that the pressurerequired to achieve a given air flow is much less for the inventiveheating element than for the ceramic heating element. For example, toachieve an air flow of 6 SCFM the comparative example requires apressure of approximately 22 pounds per square inch (psi) versusapproximately 3.5 psi for the inventive embodiment.

EXAMPLE 2

[0047] The heating elements of Example 1 were heated at 500 watts at anair flow rate of 100 SCFH. The temperature of air exiting each heaterwas monitored as a function of time using a type K thermocouplepositioned centrally in the heater exhaust and approximately 0.80″ fromthe end of the mica support or ceramic core. The thermocouple output wascaptured with a Hewlett Packard 34970A data acquisition unit linked to acomputer. The results are provided in graphical form in FIG. 7. Theefficiency of heating for the inventive heater was demonstrated to begreatly superior to that of the ceramic heater. For example, afterapproximately 5 minutes the air exiting the inventive heater wasapproximately 450° F. while the air exiting the comparative heater wasonly approximately 300° F. This increase of approximately 50%, coupledwith the lower required air pressure to achieve an equivalent air flowdemonstrates a performance which greatly exceeds that considered in theart to be achievable with a base supported coil in an in line heater.

Claimed is:
 1. An inline heating element for heating a fluid comprising: an elongated tubular heating chamber comprising an inlet and an outlet opposite to said inlet; a heating element in said heating chamber wherein said heating element comprises; an elongated base comprising a first face, a second face parallel to said first face, a first edge and a second edge; a coiled filament circumventing said base wherein said coiled filament comprises a first region wrapped clockwise around said base and a second region wrapped counterclockwise around said base and wherein said first region terminates at a first terminal and said second region terminates at a second terminal; wherein said elongated base is parallel to said elongated tubular heating element; and said fluid flows parallel to said base.
 2. The inline heating element of claim 1 wherein said base comprises at least one element selected from the group consisting of mica, steatite, cordierite, quartz and ceramic.
 3. The inline heating element of claim 2 wherein said base comprises mica.
 4. The inline heating element of claim 1 further comprising a liner between said base and said coiled filament.
 5. The inline heating element of claim 4 wherein said liner is flexible mica, mica sheet, quartz or ceramic.
 6. The inline heating element of claim 5 wherein said liner is flexible mica.
 7. The inline heating element of claim 1 wherein said filament is a resistive alloy.
 8. The inline heating element of claim 1 wherein said filament comprises a nickel chrome alloy.
 9. The inline heating element of claim 8 wherein said nickel chrome alloy comprises approximately 16-20%, by weight, chromium.
 10. The inline heating element of claim 8 wherein said nickel chrome alloy comprises approximately 35-80%, by weight nickel.
 12. The inline heating element of claim 1 wherein said filament comprises an iron chrome aluminum alloy.
 13. The inline heating element of claim 1 wherein said elongated base further comprises slots for receiving said coiled filament.
 14. An in line heating element for heating a fluid comprising: an elongated tubular heating chamber comprising a long tube side and a short tube side, an inlet and an outlet opposite to said inlet wherein said fluid enters said inlet and traverses parallel to said long tube side and exits said outlet as heated fluid; a heating element received by said heating chamber wherein said heating element comprises: a rectangular base comprising long base sides and short base sides wherein said long base sides are parallel to said long tube side; and a continuous coiled resistive heating element circumventing said base in a double helix wherein said double helix comprises a rotational axis of symmetry and said rotational axis of symmetry is parallel to said long base.
 15. The in line heating element of claim 14 wherein said base further comprises slots for receiving said continuous coiled resistive heating element.
 16. The in line heating element of claim 15 wherein said slots comprises first side slots and second side slots and said first side slots are along a first edge of said long side of said base and said second side slots are along a second edge of said long side of said base.
 17. The in line heating element of claim 16 wherein said slots are aligned.
 18. The in line heating element of claim 15 wherein a separation between slots increases with distance along said base.
 19. A heater for heating flowing fluid comprising: a cylindrical heating chamber comprising an inlet at one base and on outlet at a second base and wherein said flowing fluid traverses parallel to said chamber; a resistive heating element received in said chamber wherein said resistive heating element comprises: a rectangular base wherein said rectangular base comprises a long axis and short axis and said long axis is parallel to said chamber and said base comprises a multiplicity of first slots on one side of said rectangular base and a multiplicity of second slots on a second side of said base; and a double helix filament received in said first slots and said second slots.
 20. The heater of claim 19 wherein said first slots and said second slots are aligned.
 21. The heater of claim 19 wherein a separation between said first slots increases with distance along said base.
 22. The heater of claim 19 wherein said base comprises at least one element selected from the group consisting of mica, steatite, cordierite, quartz and ceramic.
 23. The heater of claim 22 wherein said base comprises mica.
 24. The heater of claim 19 further comprising a liner between said base and said coiled filament.
 25. The heater of claim 24 wherein said liner is flexible mica, mica sheet, quartz or ceramic.
 26. The heater of claim 25 wherein said liner is flexible mica.
 27. The heater of claim 19 wherein said filament is a resistive alloy.
 28. The heater of claim 19 wherein said filament comprises a nickel chrome alloy.
 29. The heater of claim 28 wherein said nickel chrome alloy comprises approximately 16-20%, by weight, chromium.
 30. The heater of claim 28 wherein said nickel chrome alloy comprises approximately 35-80%, by weight nickel.
 31. The heater of claim 19 wherein said filament comprises an iron chrome aluminum alloy. 